Excessive pressure generated during heating and swelling of coals in coke ovens is one of the major safety concerns for the coke and steel producers. Large pressures against coke oven walls can seriously damage the ovens and reduce their life span. In the work presented here, a laboratory scale method is used to investigate swelling pressures during slow pyrolysis of thermoplastic coals with an aim in detecting the parameters which most influence this phenomenon. One of the coals chosen for this study had dangerous swelling properties with capabilities of developing excessive pressures. Results showed significant difference in the thermal behaviour of this coal when compared with the low swelling pressure coal in the temperature region of contraction of the semi-coke. Both, the high and low swelling pressure coals, were thermally investigated using series of gas analysis and long distance microscopic techniques. Results showed similarities in the amount of gas release between the two coals, especially for lower molecular weight volatiles; however, the prime difference was in their tar evolution rates and temperatures. For this purpose, the tars were collected for both coals and their blends and further analysed using matrix assisted laser desorption ionisation mass spectrometry (MALDI). Furthermore, a long distance microscopic analysis was performed on single particles under pyrolytic heating conditions revealing significant differences in the transient structural changes between both coals, which were caused by viscosity of the plastic phase and their consequent tar content.